1. Field of the Invention
The present invention relates to silicon nitride-alumina composite ceramics suitable to the material for a heater supporting member of a ceramic glow plug, a base plate of an IC pressure sensor and the like.
2. Description of the Prior Art
.beta.'-sialon[Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (Z=0 to 4.2)] is well known as the silicon nitride-alumina ceramics.
.beta.'-sialon is a complete solid solution of silicon nitride and .alpha.-alumina, of which the coefficient of thermal expansion is as small as 3.0.times.10.sup.-6 /.degree.C. in the temperature range from room temperature to 1000.degree. C., similar to that of silicon nitride.
The material having a small coefficient of thermal expansion generally exhibits excellent thermal shock resistance, but the use thereof is limited since damage often occurs due to the difference in coefficient of thermal expansion when such material is jointed to another material such as metal.
By preparing a mixture powder of silicon nitride and alumina in an adjusted ratio and sintering the mixture powder under adjusted conditions, not complete solid solution but composite ceramics containing unreacted silicon nitride and .alpha.-alumina can be obtained. By adjusting the amount of the unreacted alumina, the coefficient of thermal expansion of the obtained ceramics can be adjusted.
However, the experimental results on many samples of ceramics containing unreacted .alpha.-alumina show that the strength thereof is scattered and some one exhibits rather low strength. This scattering of strength is not observed in the material containing only one of silicon nitride and .beta.'-sialon.